MChem Master of Chemistry Master of Chemistry: Organic Chemistry 3

Question 1

Which of the following best explains why aromatic compounds undergo electrophilic substitution rather than addition?

Accepted Answer

Substitution restores the aromatic ring's stability after electrophile attack

Answer

After the electrophile forms a sigma complex (arenium ion), loss of a proton restores aromaticity, making substitution the thermodynamically favored pathway.

Question 2

Which stereospecific reaction converts an alkene to a vicinal diol with retention of relative configuration (syn addition)?

Accepted Answer

OsO4 dihydroxylation

Answer

OsO4 adds to both faces of the double bond in a concerted fashion, delivering both hydroxyl groups from the same face (syn addition) to give a syn-diol.

Question 3

In the E2 elimination mechanism, what geometric requirement must the leaving group and β-hydrogen satisfy?

Accepted Answer

They must be anti-periplanar (180°) to each other

Answer

E2 is a concerted, stereospecific mechanism requiring anti-periplanar alignment (180°) of the H and the leaving group for proper orbital overlap.

Question 4

What is the product of treating an ester with excess LiAlH4 followed by aqueous workup?

Accepted Answer

A primary alcohol

Answer

LiAlH4 reduces esters fully to primary alcohols; the intermediate aldehyde is not isolated because it is reduced further under the reaction conditions.

Question 5

Which of the following is an example of a [3,3]-sigmatropic rearrangement?

Accepted Answer

Cope rearrangement

Answer

The Cope rearrangement is a thermal, concerted [3,3]-sigmatropic rearrangement of a 1,5-diene via a chair-like transition state.

MChem Master of Chemistry Practice Test

MChem Master of Chemistry Practice Test

MChem Master of Chemistry Master of Chemistry: Organic Chemistry 3